Retinoblastoma is a debilitating childhood cancer of the retina that can lead to impaired vision, blindness, or death. The survival rate for retinoblastoma is greater than 90% in developed countries due to early detection and comprehensive therapeutic intervention but in developing countries that lack the necessary resources, 50% of children still die of retinoblastoma. However, even with early detection and the best treatment, many children who survive retinoblastoma still suffer from reduced vision or blindness. Retinoblastoma tumors initiate with the inactivation of the RB1 gene during retinal development in utero. It has recently been reported that subsequent to RB1 gene inactivation, retinoblastoma tumors sustain genetic changes that inactivate the p53 tumor suppressor pathway. In 65% of human retinoblastomas, suppression of the p53 pathway occurs through MDMX gene amplification. The MDMX protein and a related protein called MDM2 regulate the activity of p53 and control programmed cell death during development and in cancer. When either MDMX or MDM2 are increased in tumor cells, the p53 cell death pathway is suppressed and tumor cells can rapidly expand. Therefore, if MDMX and/or MDM2 could be blocked with a drug, then the cancer cells would undergo p53-mediated cell death. We have developed a high-throughput drug screen to identify small molecule inhibitors of MDMX that will allow a better understanding of the role of MDMX in cancer cells, and may someday lead to the development of targeted chemotherapy for retinoblastoma. The goals of the experiments outlined in this research proposal are to design a small molecule inhibitor of p53. We will ultimately use the inhibitor to test the hypothesis that an inhibitor of MDMX can be identified that will lead to p53-mediated cell death in retinoblastoma cells. The long-term goal of this research is to improve the treatment of retinoblastoma, and to save vision in children with this debilitating cancer. Moreover, MDMX amplification is observed in many other cancers in addition to retinoblastoma, such as breast, colon, lung, prostate cancer, and adult B-cell leukemia. Therefore, this research may also have a broader impact, and provide new tools to improve the understanding and treatment of the wide variety of cancers that rely on MDMX. [unreadable] [unreadable] [unreadable]